1. Field of the Invention
The present invention relates to an apparatus for adjusting the optical axes of lens systems.
2. Description of the Prior Art
The size reduction of optical elements such as lenses for cameras and pick-up lenses for optical disks and the variety of aspherical lenses decrease the decentering amount permissible to one lens, and it is required that the error of the optical axis of an optical system is guaranteed to be within several microns. However, it is impossible to achieve such a guarantee only by improving the processing accuracy of parts (lenses and lens frames). As methods to achieve this guarantee by adjusting the optical axis in the assembly of lens systems, a reflection-type method and a transmission-type (point image analysis-type) method in which the disagreement of the optical axes is detected are known.
FIG. 1 is a side view of a conventional apparatus for adjusting the optical axes of lens systems. When two single lenses LS1 and LS2, for example, are attached to a lens frame 9 as shown in FIG. 1, it is necessary for the optical axes of the lenses LS1 and LS2 to coincide with each other. In assembling a high-precision lens system such as the pick-up lens for optical disks for which it is required that a lens diameter is 7 mm or less, that the total decentering after assembly is 1.degree. to 2.degree., that the total focal length after assembly is 7 mm or less and that the transmission wave front aberration is .lambda./5, it is particularly important that the optical axes of the single lenses coincide with each other.
A pinhole plate 2 having a pinhole with a diameter of approximately 1 mm is arranged to the left of a light source 1, a neutral density (ND) filter 3 is arranged to the left thereof, a collimator lens 4 is arranged to the left thereof, and a mirror 5 is arranged to the left thereof. Of the light emitted from the light source 1, the light having passed through the pinhole plate 2 passes through the ND filter 3 and the collimator lens 4 and becomes collimated light R, which is reflected by the mirror 5 and proceed downward. A pinhole plate 2' having a pinhole with a diameter of 1 mm is arranged below the mirror 5, and below it, a subject lens system T' to be adjusted is placed. The subject lens system T' is now being assembled, and while the lens LS2 has been fixed to a lens frame 9, the lens LS1 has not been fixed yet. A microscopic lens 19 is arranged below the subject lens system T'. A point image formed by the pinhole plate 2' passes through the lenses LS1 and LS2 in the lens frame 9 and is viewed through the microscopic lens 19.
The point image viewed through the microscopic lens 19 appears at the center if the optical axes of the lenses LS1 and LS2 coincide with each other and the point image is shifted from the center if the optical axes disagree. When the point image viewed is shifted from the center, the lens LS1 is slightly moved by means of a thin rod so that the point image is located at the center. After the adjustment has been completed, the lens LS1 is fixed to the lens frame 9 with an adhesive.
This method is not highly reliable; since an operator has to perform the adjustment to remove the shift of the point image from the center while viewing the point image, this adjustment requires skill, lacks mass productivity and the adjustment results differ according to the operator. Therefore, it is impossible to achieve detection precision of submicron order.
To solve this problem, Japanese Unexamined Patent Publication No. S60-150016 has proposed a method in which a photoelectric device divided into four parts which are, for example, fan-shaped with the optical axis as the center is arranged on the focal plane of a compound lens and the shift of the point image from the optical axis is photoelectrically and quantitatively evaluated by comparing the outputs from the parts of the photoelectric device. According to this method, it is possible to center and edge a predetermined lens by moving it so that the center of the point image is located to the center of division of the photoelectric device.
However, in performing the centering and edging by this method, it is necessary for the optical axis of a lens system in the compound lens which serves as a reference to coincide accurately with the center of division of the photoelectric device, and the resolution of the photoelectric device is limited. Therefore, for a lens to be mass-produced such as a photographic lens, it is necessary to adjust the optical axis for every lens, which increases the adjustment work tremendously.